1. Field of the Invention
This invention relates to a process for producing a vinyl chloride resin composition for powder molding. More particularly it relates to a process for producing a vinyl chloride resin composition for powder molding, which process, even in the case of powder molding using a mold having a complicated shape and a pattern of fine projections and depressions, affords a molded product faithfully duplicating the shape and the pattern and also having a uniform thickness.
2. Description of the Related Art
Recently, various molding processes and coating processes such as rotational molding processes, slush molding processes, fluidized bed dip coating processes, electrostatic coating processes, etc. (these molding processes and coating processes hereinafter being collectively referred to as powder molding processes) using powdery thermoplastic resin compositions have been broadly employed. Further, powdery vinyl chloride resin compositions have also been used for the powder molding.
Heretofore, as a process for producing powdery vinyl chloride resin compositions for powder molding (hereinafter referred to as powdery PVC compositions), there has generally been carried out a process wherein a powdery vinyl chloride resin, a plasticizer and various necessary additives, each in a definite quantity were introduced into a high-speed agitating mixer such as Henschel mixer (tradename), super mixer, etc. provided with a jacket for heating and cooling, followed by raising the resin temperature inside the mixer to about 130.degree. C. with stirring by passing hot water or steam through the inside of the jacket, to have the plasticizer absorbed in the powdery vinyl chloride resin, cooling the resin temperature inside the mixer down to about room temperature by passing cooling water through the inside of the jacket, then adding a definite quantity of a vinyl chloride emulsion polymer (hereinafter referred to as emulsion polymer) to the mixture inside in the mixer in order to improve the powder flowability, again agitating the mixture in the vicinity of room termperature to coat the surface of the powdery vinyl chloride resin with the emulsion polymer and taking out the resulting material to obtain a powdery PVC composition.
One of properties required for the powdery PVC composition for powder molding consists in the respective particles of the powdery PVC composition being present in discrete manner without adhering or agglomerating to one another and abundant in the powder flowability. If the agglomeration of the particles is notable, the powder flowability becomes inferior, and if such a powdery PVC composition is used, it is impossible in the powder molding using a mold having a complicated shape and a pattern of fine projections and depressions to obtain a molded product faithfully duplicating the shape and the pattern of the mold surface and having a uniform thickness, and moreover if such a powdery PVC composition is stored, the powder particles often adhere tightly to one another in a lump to lose the powder flowability and thereby be unable to use the composition for powder molding.
Thus, the powdery PVC composition obtained according to a conventional production process is still unsatisfactory in the aspect of powder flowability.
The present inventors have made extensive research on a process for producing a powdery PVC composition having improved the powder flowability. As a result, we have found that the adhesion and agglomeration of the particles among one another deteriorating the powder flowability are caused by fine particles (average particle size: 100.mu. or less) present in the powdery PVC composition; hence by reducing the quantity of such fine particles present, the powder flowability of the resulting powdery PVC composition is notably improved, and have achieved the present invention.